The present invention relates to a control method of a high voltage power source which is used for an electrostatic photographing process related to a sheet feeding system of an image forming apparatus into which an electrostatic photographing method is adopted, and specifically to a control method of the high voltage power source corresponding to the moisture content of transfer sheets.
In an image forming apparatus in which an electrostatic photographing method is adopted, the following processes are conducted: a transfer material is superimposed on a toner image which has been electrostatically carried on an image forming body; and a transfer process, in which the toner image is transferred onto the transfer material, is carried out when electric charges are discharged from the rear of the transfer material. In this transfer process, when the moisture content of a transfer sheet is more than 7% , there is a tendency to cause an imperfect transfer phenomenon, in which a toner image remains on the image forming body, and a toner smear phenomenon, in which toner slides on the transfer sheet. The reasons for these phenomena are surface resistance changes of the transfer sheet or resistance changes of the toner itself.
In order to solve these problems, the following technology has been disclosed: the moisture content of a transfer sheet is detected using the correlation between the surface resistance of the transfer sheet and the moisture content of the transfer sheet; and a discharge current of a transferring device is controlled corresponding to the moisture content (refer to Japanese Patent Publication Open to Public Inspection No. 57042/1978(hereinafter referred to as Japanese Patent 0. P. I Publication)). In this technology, the moisture content is measured as follows: a pair of conveyance rollers are provided close to a transfer area; when the transfer sheet passes through these pair of conveyance rollers, a predetermined voltage is impressed upon the sheet; and the moisture content is measured by the current which flows through the transfer sheet. Further, the following technology has been disclosed: two closed circuits, which are formed through the transfer sheet sandwiched between a pair of rollers, are provided between a transfer device and a sheet feed cassette; the surface resistance of the transfer sheet is detected by one of two closed circuits; the volume resistance of the transfer sheet is detected by another closed circuit; and a discharge current from the transfer device is controlled according to the result of these measurements (Refer to Japanese Patent Publication 0. P. I No. 28081/1980) .
The relationship between the surface resistance of the transfer sheet and the moisture content of the transfer sheet is changed according to a surface processing method of the transfer sheet. Accordingly, the surface processing method, that is, information related to various kinds of transfer sheets, is important.
In the case of the foregoing structure to detect the resistance value of the transfer sheet (Japanese Patent Publication O. P. I Nos. 57042/1978, and 28081/1980) , the transfer process can be carried out satisfactorily to some extent without being affected by the environmental temperature and humidity under which the image forming apparatus has been installed. However, the structure of a sheet feed system from a sheet feed cassette to the transfer device becomes complicated, and thereby, a control by which sheet feed timing is synchronized with voltage impression timing becomes complicated. Further, in some cases, a transfer failure or a sheet separation failure occurs.
The inventors of the present invention have studied a method for controlling the high voltage power source which is used for an electrostatic photographic process related to a sheet feeding system. The method for controlling the high voltage power source includes a method for controlling not only the transfer device but also a neutralizing device.
In this connection, the inventors have investigated the following: a change according to the elapsed time of the moisture content of the transfer sheet under high temperature and high humidity environmental conditions; and a change of the moisture content of the transfer sheet under the condition that the transfer sheet, having the saturated moisture content under the high temperature, high humidity and low temperature, low humidity environmental conditions, is left as it is.
FIG. 6 is a graph showing a change in the elapsed time of the moisture content of the transfer sheet under high temperature and high humidity environmental conditions.
In the experiment carried out by the inventors, a bundle of over 100 transfer sheets was taken from a new package of sheets and was loaded into a sheet feed cassette, and the sheet feed cassette was housed in the image forming apparatus and left undisturbed. The image forming apparatus was installed under high temperature and high humidity environmental conditions. Then, the change in the elapsed time of the moisture content of the transfer sheets loaded in the sheet feed cassette was measured. The moisture content of the transfer sheets is maintained to be approximately 6% under the conditions that the transfer sheets are packed in the package.
In the graph, the ordinate axis shows the moisture content (%), and the abscissa axis shows the elapsed time after the sheet feed cassette has been loaded into the image forming apparatus. The graph shows that there is a time lag between the humidity and the moisture content of the transfer sheet, depending on the loading sequence of the transfer sheets. Accordingly, even if the transfer sheet were the same, appropriate process conditions can not be set only when the humidity, under which the image forming apparatus has been installed, is measured, which is a problem.
In order to assume the moisture content of the transfer sheet from the ambient humidity so that the structure and control of the sheet feeding system can be simplified, it is necessary that the elapsed time after the transfer sheets have been taken out from the package is measured. The time can be measured by counting CPU clock pulses while the power source of the image forming body is turned "on" On the other hand, since the moisture content of the transfer sheet is affected by the humidity even while the power source of the image forming apparatus is turned "off", it is necessary to measure the time during this period. However, in printers or small copiers, an internal timer, which can measure the "off" time of the power source, can not be provided for economical reasons. In spite of these circumstances, in printers and small copiers, the power source is very frequently turned "on" and "off". In view of these circumstances, it is necessary to solve the problem of time measurement during the "off" period of the power source of the apparatus, considering the economical reasons, when the timer is adapted to printers or small copiers.